The present invention relates more particularly to electrostatic printing and copying, and especially to such processes utilizing liquid toners. The basic steps of the liquid toner imaging process are:
1. Generating an electrostatic latent image, e.g. on a photoconductive surface such as a plate or drum which has been sensitized by charging with a corona discharge and by exposure to light in the form of an image, the charge being dissipated in exposed areas. PA1 2. Developing the latent image by contact with a liquid dispersion ("toner") of fine particles which are attracted to the charged areas, or to the uncharged areas in a reversal type development. PA1 3. Removal of excess liquid toner and particles from the background areas. PA1 4. Transfer of the image, e.g. to a substrate such as paper. PA1 5. Fixing the image by fusing the particles together and to the substrate. PA1 6. Cleaning the plate or drum for re-use. PA1 heat; PA1 ultraviolet radiation; PA1 catalysis.
In this connection reference is made to the following published patent applications and issued patents in the field of electrophotography: GB Published Patent Applications Nos. 2,169,416A and 2,176,904A, and U.S. issued Pat. Nos. 3,990,696, 4,233,381, 4,253,656, 4,256,820, 4,269,504, 4,278,884, 4,286,039, 4,302,093, 4,326,644, 4,326,792, 4,334,762, 4,350,333, 4,355,883, 4,362,297, 4,364,460, 4,364,657, 4,364,661, 4,368,881, 4,378,422, 4,392,742, 4,396,187, 4,400,079, 4,411,976, 4,412,383, 4,413,048, 4,418,903, 4,420,244, 4,435,068, 4,439,035, 4,454,215, 4,460,667, 4,473,865, 4,480,825, 4,501,486, 4,522,484, 4,531,824, 4,538,899, 4,582,774, 4,585,329, 4,586,810, 4,589,761, 4,598,992, 4,603,766, 4,620,699, 4,627,705 and 4,678,317, the contents of all of which are incorporated by reference herein.
While certain aspects of copy quality can be associated with the efficiency of each of the above-mentioned steps of the liquid toner based electrostatic imaging process, none of the above-referenced patent applications and issued patents discloses a method of improving copy quality by a pre-transfer treatment of a developed image.
Following development in liquid toner processes, excess toner and particles are removed from the image, by a step often referred to as "metering". This step may be carried out by use of an electrically biased reverse roller. At the transfer stage, there is likely to occur a degree of smudging, smearing or squashing of the image, which de tracts from its quality. Furthermore, the entire image may not transfer completely to the carrier substrate, thus leaving a residue of toner on the surface from which the image was transferred. The quality of the transferred image also normally depends on the surface roughness of the substrate.
In Published UK Patent Application GB 2169416A (Landa et al), the contents of which are incorporated herein by reference, a degree of squash-resistance is imparted to the developed image by using fibrous toner particles prepared as follows: a thermoplastic polymer (and pigment) is plasticized with a nonpolar liquid, preferably at elevated temperature, to form a sponge, the sponge is shredded, more nonpolar liquid is added, the pieces of shredded sponge are wet-ground into particles and the grinding is continued to pull the particles apart to form fibers extending therefrom (the particles preferably having a diameter less than 5 microns). Finally, a charge director may be added to impart a charge of predetermined polarity to the toner particles.
While the method of the aforementioned Published UK Patent Application improves the transfer efficiency of the developed image and results in reduced residue on the surface in question, it would nevertheless be desirable to be able to provide an additional or alternative method for improving the transfer efficiency and to leave a negligible residue or even no residue on the surface from which the image is transferred. It is known to heat an image to melting on an intermediate transfer member, as shown, for example in U.S. Pat. No. 4,015,027. An intermediate transfer member is employed to avoid, inter alia, the undesirable alternative of heating the image on a photoconductor.